1. Field of the Invention
This invention relates to a soldering flux and more particularly, a non-corrosive soldering flux.
2. Description of the Prior Art
In soldering electronic components, circuits, equipment, etc., various kinds of fluxes are used together with soldering material so as to improve the efficiency of the soldering operation, to secure the soldered connections and to improve the long-term reliability of the connections. Conventionally, there are mainly used three kinds of flux: (1) water soluble flux which is made from inorganic acids, organic acids, amine hydrohalides, etc., (2) natural rosin and (3) activated rosin having a halogenated compound, e.g., an amine hydrohalide, incorporated with natural rosin.
Various additives have been added to rosin to form an activated rosin flux. Keto acids, for example, levulinic acid, have been combined with rosin (see U.S. Pat. No. 2,361,867). Adipic acid has also been combined with rosin in the formation of a flux core. Also, it has heretofore been known to combine adipic acid and levulinic acid with rosin to form a solder flux.
Water-soluble flux is very useful in removing oxides from the metal surface to be soldered. However, there are disadvantages in that the water-soluble flux is apt to destroy metallic material and to leave residues which corrode the soldered parts after soldering. The corrosion impairs the reliability of the soldered parts with respect to electrical and mechanical properties.
Natural rosin which is available under a name of WW rosin (water white rosin) presents no problems with respect to corrosiveness, but is inferior as a soldering adjuvant.
Activated rosin has a stability similar to natural rosin and causes little corrosion at room temperature. Also, fully activated or LRA fluxes have a strong fluxing action at a soldering temperature due to activators, such as an amine hydrochloride, which are typically present in high concentrations such as 1 to 10 weight percent of the resultant flux. However, the fully activated rosin or LRA flux has disadvantages in that a corrosive gas is produced at a soldering temperature and harms the surface of metal such as copper, brass, etc. Moreover, the residues of the activated rosin combine with moisture and produce an acid which causes corrosion similar to that produced by the water-soluble flux. Presently available fluxes containing organic hydrohalides, in the form of neutral salts, e.g., glutamic acid hydrochloride, either form corrosive metal halides at elevated temperature or the residues thereof combine with moisture at room temperature to form a corrosive acid and thus are used with possible deleterious effect for electrical soldering applications.
U.S. Pat. No. 2,898,255 reveals an activated rosin comprising a monocarboxylic acid, such as formic acid, combined with a dicarboxylic acid, such as glutaric acid. Such a flux, however, is too acidic and corrosive for practical use in electronics soldering operations. Such a flux is typical of an LRA flux which is corrosive, as evidenced by the standard copper mirror test outlined in the Electronics Industries Association Standard Number RS-402, for liquid rosin fluxes (approved Mar. 27, 1973).
A soldering flux which is superior in fluxing action and is free from corrosive action at room temperature as well as free from harmful residues is therefore desired.